ii have not tried this but it should work
did you create a new directory for your images and then point to it in the mdoule setup
make a note of the file names of each image that you want to use and the corresponding image in your thumbnail directory (they should have the same name)
make your thumbnail and name it the same name that it had and upload and over write the auto generated thumbnail
so, full size image name is stupid.jpg
make new thumbnail and name it stupid.jpg but upload it to the thumbs directory
computers are stupid, it should not know the difference
let me know if it works